Pulp is the raw material used to manufacture paper and paper products. It is produced by the mechanical and/or chemical treatment of plant substances that contain cellulose. Both types of treatments are directed at converting bulk cellulosic material, such as wood chips or the like, into pulp fibers. Mechanical treatments rely mainly on friction to separate fibers or fiber bundles from bulk material; chemical treatments generally act by eliminating constituents of the plant substances, such as lignin, that hold the fibers together.
Chemical pulp stock, i.e., pulp material produced by chemical treatment, which includes Kraft pulp and sulphite pulp stock, is often bleached to produce light colored pulp,often called "white pulp". Such bleached chemical pulp thereafter may be refined to achieve desired properties in the paper formed of such pulp. During such pulp stock refining, the pulp fibers are ground between elements, such as rotating metal plates, which crush and split the fibers, increasing fiber surface area and interfiber bonding.
Certain paper properties are dependent upon fiber surface area, and thus increase with pulp refining. For instance, where paper smoothness or paper strength are desired properties, pulp refining is continued to a significant degree and generally, the further the pulp material is refined, the smoother and stronger the paper produced therefrom will be.
Refining of chemical pulp, particularly bleached grades of such pulp, under typical commercial refining conditions, generally results in a significant loss of "brightness". The term "brightness" as used herein and as understood in the art, means the level of light reflected from a dry paper sheet formed of a given pulp, as compared to that of a standard. The higher the brightness rating, the lighter or whiter the appearance of the sheet. In many paper applications, particularly those that use bleached pulp stock in the first instance, brightness is an important paper property.
It is thus desirable to minimize brightness loss during pulp refining, particularly pulp refining of bleached grades of chemical pulp. It is desirable to provide a means for preventing such brightness loss that does not significantly add to the cost of the pulping and paper making process. It is desirable to provide a means for preventing such brightness loss that is not disruptive to such processes. It is desirable to provide such means in the form of a relatively inexpensive additive that does not interfere with such processes or with other desirable characteristics of the paper produced.